https://www.gov.uk/guidance/status-of-eu-nationals-in-the-uk-what-you-need-to-know
http://publications.iom.int/system/files/pdf/global_migration_indicators_2018.pdf
https://hansard.parliament.uk/commons/2018-03-07/debates/65828E0C-3CC7-43AE-B5D2-B5F1B5D42A4C/EuropeanUnionCitizenship
https://www.eurofound.europa.eu/sites/default/files/wpef17037.pdf